/ "Close-Up — Woman Smiling with Eyes Closed")
How common are side effects?
Most are mild, most are temporary, and most people complete their first month. Here's what the clinical data actually says.
Side Effects of Weight loss medication
/ "Close-Up — Woman Smiling with Eyes Closed")
For most patients, the most common side effects are gastrointestinal, temporary, and manageable. Most people who experience them still complete their first month of treatment. This guide gives you the honest, complete picture, helping you make the right decision for your treatment.
> 500,000 patients
> 33,000 reviews
Ongoing clinical support
Regulated by CQC
4.6 Trustpilot rating
Regulated by CEDR
No hidden costs
> 500,000 patients
> 33,000 reviews
Ongoing clinical support
Regulated by CQC
4.6 Trustpilot rating
Regulated by CEDR
No hidden costs
All content on this page has been medically reviewed by: Hassan Thwaini, Clinical Pharmacist and Copywriter, Master of Pharmacy (MPharm) on 16 April, 2026. GPhC Registration: 2221320View profile
Contents
In this guide
Mounjaro vs Wegovy: what's different?
Both are GLP-1 medications, but their side effect profiles aren't identical. The differences are subtle, but they can matter for the right patient.
What to expect, week by week
Side effects aren't random. They follow a predictable pattern, and knowing what's coming makes them significantly easier to manage.
Managing common side effects
Nine evidence-based strategies, from meal timing and hydration to what to tell your pharmacist. The same advice our clinicians give every day.
Serious side effects
For the vast majority, side effects are manageable. But some symptoms require you to stop your medication and seek help.
Long-term safety
Over 40% of people worry about organ impact. Here's what years of clinical trial data, including 17,000-person cardiovascular studies, actually shows.
Knowing where to look
It's hard to know what to believe about weight loss medications. Social media is full of extreme stories, headlines can't decide if it's a miracle or a risk, and there's no shortage of people with opinions about whether you should be taking it at all. If you've ended up more confused than when you started looking into it, that's not on you — it's a noisy space.
One thing worth knowing: these medications aren't as new as they can feel. Semaglutide has been prescribed since 2017. The major trials involved thousands of people over several years.1
The statistics
You're not alone
If you're worried about side effects, you're in good company. Our data shows it's the number one thing holding people back from starting a weight loss programme.2-5
of people considering GLP-1 medications are concerned about internal organ impact
of people say that they're concerned about bone density side effects
of patients who experience side effects still complete their first month
What are weight loss medications and why do they cause side effects?
If you've been researching weight loss medication, you've probably come across some worrying side effect statistics. Before those numbers cause unnecessary worry, it helps to understand why these effects occur and what the clinical evidence actually shows about their frequency, severity, and duration.
GLP-1 receptor agonists: semaglutide (Wegovy)
GLP-1 medications mimic a gut hormone that slows digestion, signals fullness, and reduces appetite. That same mechanism - slowing food through your digestive system - is what causes nausea, constipation, and bloating. It's your body adapting to a new hormonal signal. That adjustment takes time.6
Dual receptor agonist: tirzepatide (Mounjaro)
Mounjaro activates both GLP-1 and glucose-dependent insulinotropic polypeptide (GIP) receptors. Some evidence suggests the GIP component may partially counteract abdominal discomfort, which may explain why some patients report fewer gastrointestinal side effects with Mounjaro than with Wegovy, though individual responses vary considerably.7
Lipase inhibitor: orlistat (Alli)
Rather than acting on gut hormones, Orlistat blocks the enzyme lipase, preventing around a third of dietary fat from being absorbed. The unabsorbed fat passes through, which is why its side effects - oily stools, urgency, and flatulence - are entirely distinct and directly proportional to how much fat you eat.8
Side effect frequency
How common are the side effects?
Very few people tend to stop taking semaglutide (4.5%) and tirzepatide (6.2%) as a result of gastrointestinal symptoms.4,5 The frequency categories below follow the standard clinical classification used in Summary of Product Characteristics (SmPC) documents and NICE guidelines, which is the same framework your prescriber uses.9-11
| Frequency | Definition | Examples |
|---|---|---|
| Very common (>1 in 10 people) | More than 10% of patients will experience these | Nausea, diarrhoea, vomiting, and constipation (Wegovy and Mounjaro); oily stools, flatulence, and urgency (orlistat) |
| Common (1 in 10 to 1 in 100) | Between 1% and 10% of patients | Abdominal pain or discomfort, headache, fatigue, dizziness, sulfur burps (eructation), and injection site reactions |
| Uncommon (1 in 100 to 1 in 1,000) | Less than 1% of patients | Gallstones, acute pancreatitis, hair loss, and raised heart rate |
| Rare (<1 in 1,000) | Very few patients | Nonarteritic anterior ischaemic optic neuropathy (NAION), and severe hypersensitivity reactions |
Mounjaro vs Wegovy side effects
Both Wegovy and Mounjaro are GLP-1-based medications, so their side effect profiles share significant overlap. The differences, while clinically meaningful, are often subtle, and individual responses vary considerably. The most important thing to know is that neither medication is categorically "easier" for every patient:4,5,9-11
Very common side effects
Nausea: Reported by up to 31% of participants at the 15mg maintenance dose in SURMOUNT-1. Typically most pronounced in the first 1–4 weeks and after each dose increase. In most cases mild to moderate in severity.
Diarrhoea: Affects approximately 23% of patients. Usually self-limiting and resolves within a few days of each dose adjustment.
Constipation: More than 1 in 10 users, particularly in early weeks. Caused by reduced gut motility.
Vomiting: Reported in around 12% at higher doses. Most common when eating too quickly or in excess of appetite signals.
Common side effects
Abdominal pain and discomfort: Stomach cramping or bloating, usually mild. Often coincides with nausea.
Eructation (sulfur burps): One of the more socially disruptive symptoms; caused by slowed gastric emptying allowing hydrogen sulphide to accumulate. Manageable with dietary changes.
Fatigue: Especially noticeable in the early weeks, partly due to reduced caloric intake and the body's metabolic adjustment.
Headache: Often linked to dehydration and reduced food intake rather than a direct drug effect.
Dizziness: Can result from reduced appetite leading to lower blood sugar or blood pressure.
Warning signs
Acute pancreatitis: Severe abdominal pain radiating to the back, with or without vomiting. Stop medication and seek urgent medical care.
Cholelithiasis (gallstones): Right upper quadrant pain, jaundice, or fever. GLP-1 medications increase gallstone risk due to altered bile composition and reduced gallbladder motility.
NAION (vision loss): Sudden loss of vision in one eye which can affect around 1 in 10,000 people. A rare association noted in observational studies; mechanism not fully established. Discuss pre-existing eye conditions with your prescriber.
Very common side effects
Nausea: The most frequently reported side effect, affecting up to 44% of participants in STEP-1, though the majority of cases were mild or moderate. Nausea rates are broadly similar to tirzepatide at equivalent stages of dose escalation.
Diarrhoea: Affects approximately 30% of patients. Tends to be most prominent in the early weeks.
Vomiting: Reported in around 24% at maintenance dose. Eating too quickly or ignoring satiety cues is the most common precipitant.
Constipation: Affects more than 1 in 10 users, often alternating with diarrhoea in some patients.
Headache: Very common; particularly prevalent in the first month. Closely linked to fluid and calorie intake.
Common side effects
Fatigue: Particularly notable in weeks 1–6. Often improves significantly as weight loss progresses and dietary habits stabilise.
Abdominal pain and bloating: Reflux symptoms are reported more frequently with semaglutide than with tirzepatide in some head-to-head analyses.
Dizziness: As with tirzepatide, predominantly related to reduced food and fluid intake.
Injection site reactions: Minor redness or bruising at injection site; rotate between thigh, abdomen, and upper arm.
Warning signs
Acute pancreatitis: Severe abdominal pain radiating to the back, with or without vomiting. Stop medication and seek urgent medical care.
Cholelithiasis (gallstones): Right upper quadrant pain, jaundice, or fever. GLP-1 medications increase gallstone risk due to altered bile composition and reduced gallbladder motility.
NAION (vision loss): Sudden loss of vision in one eye which can affect around 1 in 10,000 people. A rare association noted in observational studies; mechanism not fully established. Discuss pre-existing eye conditions with your prescriber.
What to expect, week by week
Side effects of weight loss medications aren't random. For the majority of patients, they follow a broadly predictable pattern which keeps us in the know of what's coming, and when. This also helps reduce anxiety, supports adherence, and helps you and your prescriber get over any hurdles.
The timeline below reflects the typical pattern seen in clinical studies and in Numan’s clinical experience with GLP-1 and dual agonist medications.
Weeks 1-4: getting started
The first two weeks are the most challenging. Nausea, reduced appetite, and fatigue are most common, often described as a mild stomach bug. By weeks three and four, nausea eases, cravings quieten, and for many people the medication starts to feel like it's working.
Weeks 5-8: dose increase
Both semaglutide and tirzepatide increase gradually every four weeks.¹⁰'¹¹ Each step up can briefly revive earlier symptoms - usually nausea or constipation - but the second adjustment is almost always shorter and milder than the first. If it's affecting your quality of life, your prescriber can slow the pace.
Weeks 9-12: ongoing treatment
For most patients, this is when the medication settles into a steady routine. Appetite suppression is more consistent, and energy levels often recover as weight loss progresses and metabolic health improves.
Management
How to manage the most common side effects
The following management strategies are based on clinical guidance and the evidence available for each medication. These are the same recommendations Numan prescribers give patients every day. Where they don't work, we adjust your dose, timing, or approach.
Eat smaller, slower
Eat smaller portions, more slowly, and stop when you feel full, well before you would have done before starting medication.14
Stay hydrated
Dehydration significantly worsens nausea and constipation. Aim for 2–2.5 litres daily.
Inject in the evening
Consider injecting in the evening so that the peak pharmacological effect occurs during sleep.
Eat some ginger
Ginger, in the form of tea, lozenges, or capsules, has evidence supporting its use as a mild antiemetic and is safe to use alongside GLP-1 medications.15
Move after meals
Light movement stimulates gut motility and can significantly reduce constipation and bloating.
Visit a pharmacist
Certain laxatives available over-the-counter can be safe to use alongside GLP-1 medications when used short-term if you suffer from constipation.15,16
Avoid high-sulphur foods
Eggs, cabbage, broccoli, garlic, and onions worsen sulpher burps - cut back in the early weeks.
Prioritise protein
Don’t undereat, and prioritise protein, aiming for at least 1.2g per kg of bodyweight daily.20 Protein preserves muscle mass during weight loss and has a significant impact on energy levels and satiety.21
Bloods and caffeine
Check iron and B12 levels if fatigue persists beyond 6 weeks.18 Avoid excessive caffeine, which can disrupt sleep quality and worsen fatigue over time.19
Recognising serious side effects
When taken as prescribed, the side effects of weight loss medications are, for the overwhelming majority of patients, mild and manageable. However, all licensed weight loss medications carry a small risk of more serious effects.
All patients taking licensed weight loss medications in the UK can report suspected adverse events directly to the Medicines and Healthcare products Regulatory Agency (MHRA) via the Yellow Card Scheme at yellowcard.mhra.gov.uk. This reporting system is how new or unexpected side effects are identified post-approval, and your report, however minor, contributes to the ongoing safety monitoring of these medications.
Emergency symptoms that require termination of treatment
Stop medication and seek immediate medical help:9,10
Acute pancreatitis: Sudden, severe abdominal pain that radiates to your back, with or without vomiting. This is a medical emergency. Stop your medication and go to A&E or call 999.
Anaphylaxis: Difficulty breathing, swelling of the face, lips, tongue, or throat. Call 999 immediately.
NAION (vision loss): Sudden loss of vision or significant blurring in one or both eyes. Stop medication and seek an emergency ophthalmological assessment.
Symptoms that warrant a call to your prescriber (non-emergency)
Severe or persistent vomiting that prevents adequate food and fluid intake for more than 48 hours.
Jaundice (yellowing of the skin or whites of the eyes), which may indicate gallbladder or liver involvement.
Persistent tachycardia (fast heartbeat) that is new or unusual for you.
Side effects at any stage that are significantly impacting your quality of life.
Managing orlistat's side effects
Orlistat's mechanism produces an entirely different side effect profile. Rather than acting on gut hormones, it prevents fat digestion, meaning unabsorbed fat passes through the gastrointestinal tract. The result is oily stools, faecal urgency, increased bowel frequency, and flatulence, all directly correlated with how much fat you eat.8
Unlike GLP-1 medications, these effects don't ease over time. They remain diet-dependent throughout treatment, which, for many patients, becomes a useful behavioural prompt. The most effective management strategy is to keep dietary fat below 30% of daily calories, spread evenly across meals. Avoid high-fat meals entirely in the early weeks while you find your individual threshold.8
One additional step is to take a daily multivitamin containing fat-soluble vitamins A, D, E, and K at bedtime, separated from your orlistat dose. Orlistat reduces their absorption, and supplementing prevents deficiency over time.8
Long-term safety
Over 40% of people considering weight loss medication worry about the impact on their internal organs. It's one of the most common concerns we hear, and it deserves a direct answer.
What the clinical data shows:
Heart health: The SELECT trial followed 17,604 people on semaglutide for nearly three years. It found not just sustained weight loss, but a statistically significant reduction in major cardiovascular events. For high-risk patients, the evidence points to cardioprotection.22
Thyroid concerns: Animal studies raised a flag. Human data hasn't. There's no established causal link between GLP-1 medications and thyroid cancer in humans. The MHRA and NICE continue to monitor this actively, and prescribers screen for relevant risk factors before treatment begins.23
Tirzepatide: Long-term SURMOUNT data is still accumulating, and the emerging picture is consistent with semaglutide's profile.10
Orlistat: The longest track record of all. In use in the UK since the late 1990s, with decades of real-world safety data behind it.8
We don't just prescribe, we supervise
The medication was never the hard part. The hard part is the months where side effects make you question whether this was the right decision; when the dose increases and symptoms return, and no one has told you that's normal. That's where most providers go quiet. That's where we stay.
Every Numan patient has a UK-registered prescriber monitoring their treatment at every step. A clinician with authority over your prescription, who can adjust your dose, slow your escalation, or change your approach if something isn't working.
What supervision actually looks like
Shame narratives around weight loss medication are real, pervasive, and harmful. At Numan, your clinical team's only interest is your outcome - not your route to it, not your history with it, and not how long it takes. Commitment with Numan runs in both directions, and it starts with supervision.
Dose monitoring at every stage
Your prescriber reviews your progress before any dose increase - not after.
Messaging through the app
Ask questions, report symptoms, and get clinical or coaching responses when you need them.
Flexible dose titration
Struggling at any stage? We can pause, reduce, or adjust your timing. No additional cost.
Proactive side effect support
We share what to expect before each dose change, so you're prepared, not surprised.
Long-term planning
We care about your weight even after you stop medication, so we build the transition plan into treatment from the start.
UK DOCTORS AND CLINICIANS
Your expert team
Specialists in medicine, nutrition, performance, and diagnostics.
Danielle Brightman
Clinical Director
MPharm PgDip PCert
Zoe Griffiths
VP of Behavioural Medicine
BSc (Hons) RD SCOPE
Shivani Sharma-Savani
Obesity Clinical Lead
MPharm PGCert PCert IP
Faye Townsend
Coaching Operational Lead
AfN BSc BDA SENr (Registered Nutritionist)
Jess Uffindell
Registered Nutritionist
BANT CNHC BSc (Hons)
Victoria Rogers
Head of AI Coaching and Behavioural Science
MSc BSc
Dr Aisha Jinnah
Numan Doctor
BSc MBBS MRCGP
Dr Michael Lacey
Numan Doctor
MBChB BSc (Hons) MRCGP
Dr Alexandra Davidson
Numan Doctor
BSc (Hons) MBBS MRCGP AFHEA MA (Hons) AFMCP
Dr Dimitris Schizas
Numan Doctor
MBBS MRCGP MSC BSSM
FAQS
Your questions answered
Knowledge
Side effects: what you need to know
References
Aamir AB, Latif R, Alqoofi JF, Almarzoq FA, Fallatah JO, Hassan GA, et al. Comparative efficacy of tirzepatide vs. Semaglutide in reducing body weight in humans: A systematic review and meta-analysis of clinical trials and real-world data. J Clin Med Res. 2025;17(5):285–96.
Wegovy® mechanism of action. novoMEDLINK.
Internal data
Jastreboff AM, Aronne LJ, Ahmad NN, Wharton S, Connery L, Alves B, et al. Tirzepatide once weekly for the treatment of obesity. N Engl J Med. 2022;387(3):205–16.
Wilding JPH, Batterham RL, Calanna S, Davies M, Van Gaal LF, Lingvay I, et al. Once-weekly semaglutide in adults with overweight or obesity. N Engl J Med. 2021;384(11):989–1002.
Ghusn W, Hurtado MD. Glucagon-like Receptor-1 agonists for obesity: Weight loss outcomes, tolerability, side effects, and risks. Obes Pillars. 2024;12(100127):100127.
[accessed 8 Apr 2026] Available from: https://www.novomedlink.com/obesity/products/treatments/wegovy/about-wegovy/how-wegovy-works.html
What is Mounjaro? Uses, How it Works and Side Effects. Diabetes UK. 2024. [accessed 8 Apr 2026] Available from: https://www.diabetes.org.uk/about-diabetes/looking-after-diabetes/treatments/tablets-and-medication/glp-1/mounjaro
Bansal AB, Patel P, Al Khalili Y. Orlistat. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2026.
Wegovy 0.25 mg, FlexTouch solution for injection in pre-filled pen. Org.uk. [accessed 8 Apr 2026] Available from: https://www.medicines.org.uk/emc/product/13799/smpc
Mounjaro KwikPen 10mg solution for injection in pre-filled pen. Org.uk. [accessed 8 Apr 2026] Available from: https://www.medicines.org.uk/emc/product/15484/smpc
Alli 60 mg hard capsules. Org.uk. [accessed 8 Apr 2026] Available from: https://www.medicines.org.uk/emc/product/6533/smpc
Gorgojo-Martínez JJ, Mezquita-Raya P, Carretero-Gómez J, Castro A, Cebrián-Cuenca A, de Torres-Sánchez A, et al. Clinical recommendations to manage gastrointestinal adverse events in patients treated with glp-1 receptor agonists: A multidisciplinary expert consensus. J Clin Med. 2022;12(1):145.
Ernst E, Pittler MH. Efficacy of ginger for nausea and vomiting: a systematic review of randomized clinical trials. Br J Anaesth. 2000;84(3):367–71.
Ihara E, Manabe N, Ohkubo H, Ogasawara N, Ogino H, Kakimoto K, et al. Evidence-based clinical guidelines for chronic constipation 2023. Digestion. 2025;106(1):62–89.
Disney B, Trudgill N. Managing a patient with excessive belching. Frontline Gastroenterol. 2014;5(2):79–83.
Ardavani A, Aziz H, Smith K, Atherton PJ, Phillips BE, Idris I. The effects of very low energy diets and low energy diets with exercise training on skeletal muscle mass: A narrative review. Adv Ther. 2021;38(1):149–63.
Sievenpiper JL, Ard J, Blüher M, Chen W, Dixon JB, Fitch A, et al. Nutritional and lifestyle supportive care recommendations for management of obesity with GLP-1 - based therapies: An expert consensus statement using a modified Delphi approach. Obes Pillars. 2026;17(100228):100228.
Kiani AK, Dhuli K, Donato K, Aquilanti B, Velluti V, Matera G, et al. Main nutritional deficiencies. J Prev Med Hyg. 2022;63(2 Suppl 3):E93–101.
Gardiner C, Weakley J, Burke LM, Roach GD, Sargent C, Maniar N, et al. The effect of caffeine on subsequent sleep: A systematic review and meta-analysis. Sleep Med Rev. 2023;69(101764):101764.
Lincoff AM, Brown-Frandsen K, Colhoun HM, Deanfield J, Emerson SS, Esbjerg S, et al. Semaglutide and cardiovascular outcomes in obesity without diabetes. N Engl J Med. 2023;389(24):2221–32.
Capuccio S, Scilletta S, La Rocca F, Miano N, Di Marco M, Bosco G, et al. Implications of GLP-1 receptor agonist on thyroid function: A literature review of its effects on thyroid volume, risk of cancer, functionality and TSH levels. Biomolecules. 2024;14(6):687.
Herrera HO, Bordeaux JS. Risk of new-onset hair loss with semaglutide and tirzepatide: A TriNetX cohort study. J Am Acad Dermatol. 2026;
Trüeb RM. ‘let food be thy medicine’: Value of nutritional treatment for hair loss: Value of nutritional treatment for hair loss. Int J Trichology. 2021;13(6):1–3.
Yu W-S, Huang J-Y, Lo S-C, Huang C-N, Yang Y-S, Kornelius E. Association of tirzepatide and the risk of suicide in a real-world cohort. Front Psychiatry. 2025;16(1626103):1626103.
Medically reviewed: